The present invention refers to semisubmersible offshore vessels having parallel twin underwater hulls, provided with propulsion machinery comprising a number of azimuth thruster propellers.
Each thruster propeller is rotatable 360.degree. in relation to a vertical axis, and is used for propulsion as well as for steering. A very important function of these propellers is to retain the vessel in a desired working position during operation.
These thruster propellers are necessarily mounted below the bottom plating of the hulls.
During a voyage the action of wind, waves and streams may make it necessary to direct one or more propellers at one of the hulls, so the water jet, or jets will be directed towards the other hull. Such a directing of one or more propellers will be especially noticeable, when it is desirable to retain the vessel in a desired position. The portion of a water jet hitting the side of the adjacent hull will of course apply a force upon the latter, which causes an undesirable steering action.
The fact that the thruster propellers will project from the bottom plating of the hulls will cause problems during docking and when the vessel is brought into a harbour with a limited water depth.